The present invention relates to a Fresnel lens and its production process. This lens is designed to function in integrated optics structures.
Remote data transmission and/or processing methods which have been studied for several years use transmission by light waves in light guides having a planar structure. These methods are called "integrated optics".
A simple light guide generally comprises a substrate with a refractive index n.sub.s covered with a guidance layer having a real refractive index n.sub.g, which is generally higher than the refractive index of the substrate. The structure is completed by air, whose refractive index is below the real refractive index of the guidance layer. In the case of such a guide an effective index n.sub.eff =c/v is defined in which v represents the propagation speed of the light in the waveguide and c the propagation speed of the light in vacuum.
As the value of the effective index depends on the value of the different indices of the layers constituting the light guide, as well as their thickness, in integrated optics the speed of a light wave can be modified either by varying the index or by varying the thickness of the various layers used.
By transposing phenomena known in conventional optics attempts have been made to construct in the form of integrated components structures equivalent to the conventional structures for permitting the propagation of light. An integrated lens was one of the first components which it was attempted to produce.
The presently known lenses are of the geodesic type and such a lens is shown in FIG. 1.
This lens is constituted by a substrate 2, e.g. of lithium niobate in which, after forming a light guide 4, a depression 6 having a perfectly defined geometry is made. By the very fact that depression 6 is present this lens has a certain number of technical disadvantages.
In addition, the methods used for producing this lens (ultrasonic micropolishing) are not compatible with the production methods of the other components which it is wished to associate with the lens to obtain more or less complex optical systems, such as a spectral analyzer. The main methods used for producing these components are photolithography, chemical etching, etc. In particular it is difficult to position the lens relative to the various associated components. It is also very difficult to prevent defects relative to the edges 8 of the lens of FIG. 1. These defects lead to a significant light diffraction, which is generally prejudicial to the quality of the lens.
The invention therefore relates to an integrated lens and its production process making it possible to obviate these disadvantages. This lens is of the Fresnel type.
A Fresnel lens is governed by the association of two elements, namely a diffracting plane and a particular phase displacement associated with each point of the diffracting plane such that for a point F of the waveguide for which the lens is produced, all the diffracted light beams which converge at this point occur in constructive interferences. Point F constitutes the focus of the lens.
In conventional optics the Fresnel lens shown in perspective view in FIGS. 2a and 2b can be formed by a sequence of rings constituted by portions of meniscus lenses 10, which can either be plano-convex 10a or plano-concave 10b. The portions of the meniscus lenses 10 have a length L(r) dependent on the distance r from the axis 11 of the lens. The phase displacement produced by such structures varies according to the thickness of the material traversed, i.e. the phase displacement is dependent on the length L(r) of the meniscus lens portions.
In general terms if p and p' are distances between the object points and the image we obtain: ##EQU1## if the variation of index .delta.n is positive for a convergent lens and if .delta.n is negative for a divergent lens, and ##EQU2## if the variation of the index .delta.n is negative for a convergent lens and if .delta.n is positive for a divergent lens.
In the particular case of an object point at infinity p' is equal to the focal distance f of the lens and the formulas become: ##EQU3## in the first case (FIG. 2a) and ##EQU4## in the second case (FIG. 2b).
In these formulas f represents the focal distance of the lens, .lambda..sub.0 the wavelength in the vacuum of the radiation used and m an integer representing the rank of the ring in question. The integer m is increased by one unit on each occasion that L(r) is cancelled out, i.e. on passing from one ring to the next. Rank m can be taken as equal to 1 for r=0, which gives a length L(r)=L.sub.0 at the centre.
In both cases the length L.sub.0 is taken in such a way that there is a phase displacement of 2.pi. on the axis of the lens, i.e. ##EQU5## with .lambda. the wavelength used.
It should be noted that in these formulas the length L(r) of the portions of the meniscus lenses 10 is defined to within L.sub.0, i.e. to within 2.pi..